1. Field of the Invention
This invention relates to improvements in methods and apparatuses for decoding noisy, intermittent data, such as Manchester encoded data, or the like, and additionally relates to methods and circuits having multiple modes of operation depending upon the data signal that is received.
2. Relevant Background
Manchester encoded data is useful for reliably transmitting telemetry and other types of data. Typically, for example, a Manchester encoded data stream may be generated from an encoded telemetry data stream, which may be, for example, a binary nonreturn-to-zero (BNRZ) encoded signal (or data stream encoded by another similar technique), known in the art. Upon receiving the Manchester encoded signal, the signal is decoded to recover the original BNRZ encoded signal. One of the problems inherent in data transmission by any means, especially via radio frequency transmissions, is that the signal becomes noisy, accumulating static, or other rf signals or noise. This makes decoding the Manchester signal difficult to reliably perform.
Manchester encoding, which is widely used in data transmission and telemetry fields, defines data states of the signal to be encoded by the direction of midpoint transitions in an encoding signal, which will become the Manchester encoded data stream. The Manchester encoded data stream has time sequential "cells" of equal duration. At the midpoint of each cell, the data changes state in a direction that indicates the state of the signal to be encoded.
Thus, for example, a transition from a high to low logic state indicates that the signal to be encoded is in a logic low state. On the other hand, a transition from a low to high logic state indicates that the signal to be encoded is in a logic high state. Of course, at the end points of each cell, the state of the signal that will form the Manchester encoded data stream must be set up or established to enable the next midpoint transition. Thus, if a logic zero is to be encoded, the signal that will form the Manchester encoded data stream must be in an initial logic high state so that the midpoint transition from high to low can be realized. Alternatively, if a logic one is to be encoded, the signal that will form the Manchester encoded data stream must be in an initial logic low state so that the midpoint transition from low to high can be realized.
It can therefore be seen that if a series of logic states that are the same are encoded, the resulting Manchester encoded signal will be a square wave of period equal to the length of the cell. On the other hand, if a series of alternate logic ones and zeros are to be encoded, the resulting Manchester encoded signal will be a square wave of period equal to twice the length of the cell.
Various methods for decoding Manchester encoded data have been proposed. One popular technique is to use a phase locked loop circuit. In practice, however, sometimes a Manchester encoded signal is formatted to provide a "wake up" sequence, such as ten data cells, followed by a short dead time, followed by the actual data. Since the wake up sequence is so short, only 10 data cells, the circuit might not lock and might drift during the short dead time. Thus, the commonly used phase locked loop decoding technique cannot be used.
Other decoding techniques employ analog and digital matched filters, integrate and dump schemes, and highly over sampled digital signal processing techniques. Long synchronization time and high component count preclude the use of most of these schemes.
One method that has been proposed employs a gating circuit that responds to the mid-cell transitions in a Manchester encoded waveform to produce an enabling signal. The enabling signal causes a clock circuit to generate high frequency clock pulses, which are accumulated in a programmable counter. If the counter exceeds a clock count threshold before the beginning of the following enabling signal, a storage element is caused to sample and store the encoded waveform.